1. Field of the Invention (Technical Field)
The present invention relates to methods and apparatuses for controlling remelting processes, namely vacuum arc remelting (VAR) and electro-slag remelting (ESR) processes.
2. Description of Related Art
In the VAR process, a cylindrically shaped, alloy electrode is loaded into the water-cooled, copper crucible of a VAR furnace, the furnace is evacuated, and a direct current (dc) arc is struck between the electrode (cathode) and some start material (e.g., metal chips) at the bottom of the crucible (anode). The arc heats both the start material and the electrode tip, eventually melting both. As the electrode tip is melted away, molten metal drips off and an ingot forms in the copper crucible. Because the crucible diameter is larger than the electrode diameter, the electrode must be translated downward toward the anode pool to keep the mean distance between the electrode tip and pool surface constant. The speed at which the electrode is driven down is called the electrode feed rate or drive speed. The mean distance between the electrode tip and the ingot pool surface is called the electrode gap.
Pool power control involves simultaneously controlling the electrode feed rate and melting current to regulate the energy flux to the ingot pool surface. The fraction of total power consumed by melting is given by fm. Under the assumption of steady-state thermal conditions in the electrode, conduction losses along the electrode may be neglected because the electrode burn-off rate matches the rate at which the thermal boundary layer front propagates up the electrode. Neglecting radiation, the melting power is equal to the total power absorbed from the arc plasma by the electrode tip due to electron and ion impacts. This power is input to the ingot pool as heat contained in the dripping metal. The pool also receives power from the arc plasma. Thus, under nominal, steady-state conditions, total power to the ingot surface is approximated byPpool≈{dot over (M)}hM,sup+f(1−fm)IVwhere f is the fraction of the arc plasma power collected by the ingot surface, I is current, V is voltage, {dot over (M)} is electrode melt rate and hM,sup is the mass specific enthalpy at superheat temperature contained in the metal dripping from the electrode tip. Under nominal processing conditions for Alloy 718, ˜85% of the total power available is collected by the pool as arc power and melt power, the rest being collected as arc power by the crucible wall above the ingot surface.
The present invention provides a method and apparatus to control a VAR (or ESR) process by controlling Ppool during it. Current state-of-the-art VAR controllers seek to control electrode gap and melt rate. However, melt rate control during transient melting conditions allows for large excursions in melt power and, therefore, power delivered to the ingot pool surface. This, in turn, causes variations in the solidification rate of the electrode which may lead to the formation of solidification defects in the ingot.